Field of the Invention
The present invention relates to anti-pollution devices for internal combustion engine exhaust, and more particularly to filters that utilize catalytic beds, through which the exhaust gases pass, in order to filter the gas for particulates and to complete the combustion process, eliminating many of the harmful combustion byproducts from the exhaust stream before discharge to the environment.
Description of the Prior Art
Internal combustion engines, whether diesel or gasoline, generally operate at fuel-to-air mixtures that are greater than stoichiometric. The resultant exhaust gases contain varying amounts of partial combustion products, such as carbon monoxide, unburned hydrocarbons, nitrogen oxides, particulates, and other minor constituents. Almost all of these by-products have an adverse impact on the environment. Beginning in the 1960's, much work has been done in an attempt to reduce or eliminate these by-products from the exhaust stream.
Although much effort has been made to improve the efficiency of the combustion process itself, it remains necessary to supply more fuel for combustion than would be necessary if perfect mixing and burning occurred. To meet the increasingly more stringent clean air statutes, it has therefore also been necessary to provide a post-combustion treatment of the exhaust gases.
Initial efforts in post-treatment have focused on known techniques, such as direct flame oxidation of exhaust gas and the use of cyclone separators and filters for removing particulates. As a practical matter, these devices are more useful in the commercial and manufacturing setting, and are too complex and costly for incorporating into the primary sources of exhaust gases, automobiles and trucks. In recognition of these limitations, present technology and future development work have both focused on the use of catalytic mufflers and converters for oxidizing the various exhaust byproducts.
Diesel engines are more efficient and complete-burning than gasoline engines. Clean-air regulations have to date not included the emissions from diesel engines within their area of impact. However, the particulate and nitrous oxide discharge levels of diesel engines pose an increasingly difficult problem as their popularity has grown, and it is expected that diesel engines will soon also be subject to emission reduction requirements. Diesel emissions have always posed a problem in underground mines, and various exhaust gas filters have been proposed to remove such emissions, for example the filters of Bagley, et al., U.S. Pat. No. 2,789,032, and of Forry, U.S. Pat. No. 2,911,289.
The mufflers and converters used with gasoline-powered engines also have adopted a wide variety of shapes and mechanisms for operation. The straight-forward approach is shown in Stiles, U.S. Pat. No. 3,220,794, wherein the cylinder through which the exhaust gases are passed is solidly packed with a catalytic material. Other systems create a network of baffles and passageways, such as is shown by Greipel, et al., U.S. Pat. No. 3,365,863, through which the exhaust gases pass as part of the cleansing process. The resistance to air flow caused by the packing material and the circuitous baffle passageways result in the generation of significant pressure drops through the exhaust device. Such resistance to air flow causes "back pressure" to applied to the engine, resulting in an adverse effect on engine performance.
In order to minimize the flow resistance through the exhaust filter and muffler, a central tube formed by the catalyst-supporting substrate is provided in Barber, et al., U.S. Pat. No. 3,495,950 (Col. 8, Lines 60-74). The catalytic material is supported by a mass of fibrous material that has been twisted to impart a helical configuration to the mass. A portion of the gases flowing through the linear passageway is diverted to flow through the helical, filtering pathways of the fibrous material. To increase the amount of gas diverted through the helical pathways, the fibrous material is formed into sections with alternating twists applied to adjacent sections, providing contrasting helix angles to disrupt the air flow.
More positive mechanisms for distributing exhaust gases through a muffler or converter have also been used. For example, fans and various other types of impeller mechanisms have commonly been used in connection with exhaust gas treatment, either to assist in the actual filtering process, or as merely energy dissipation devices as in Collier, Jr., U.S. Pat. No. 2,372,765. In Muckley, U.S. Pat. No. 3,376,695, a fan 34 is rotated by the flow of exhaust gases, and is used to direct the gases towards a removable filter cartridge 22. Here also, the extent to which the flowing exhaust gases can be used to generate the motive power for forcing the gases through a filter is governed by the amount of back pressure permissible before engine performance suffers. To lessen the back pressure problem, Kile, U.S. Pat. No. 3,593,499, utilizes a powered fan located either before or after the filter element to push (or pull) the exhaust gases through the filter element.
The spinning turbine provided in Court, U.S. Pat. No. 2,792,909, is used to both spin a centrifugal filter element and to deflect the exhaust gases into an outlet 6. A similar type of deflection and rotational generating system is utilized by Urban, U.S. Pat. No. 4,047,895, in which the rotation produced by the flowing exhaust gases is used to rotate the filter medium through a reconditioning mode to prolong the life of the filter system.
In each of the fan-powered systems, there is essentially only one gas flow path with a gas treatment structure placed somewhere therein. Thus, even the fan-powered systems have the back pressure problems associated with restricted flow paths. This is particularly the case where the velocity of the exhaust gases is used to power the impeller. Moreover, even when an outside power source is utilized, the concentration of gas treatment at a single location rather over an extended area as in the Barber, et al., muffler, can lead to plugging problems in addition to mass and heat transfer problems due to the number of chemical reactions forced to occur over a limited area.